Melt blowing apparatus with parallel flow filament attenuating slot

ABSTRACT

A melt blowing die for extruding filaments of a polymer by a suitable configured air supply system to provide critical influencing and control over the molecular orientation, crystallinity and crystal orientation in high speed fiber spin line applications. Control of the both the magnitude and location of the applied shearing force is provided, through the design characteristics of the air supply system and, in particular, the attenuation of the filament through an attenuation slot; in one form in conjunction with the introduction of the air flow to the filament in a parallel flow caused by a Coanda bend in a second form in conjunction with a properly designed internal channel.

FIELD OF THE INVENTION

[0001] This invention relates to a melt blowing die apparatus forspinning filaments of molten synthetic fiber material to produce fibrousnon-woven thermal insulating mats constructed of thermoplastic fibersand particularly, though not exclusively, to form high loft batts oflinear condensation polymers, preferably polyester, for example,polyethylene terephthalate (PET). The mats may be thermally insulatingmats in the form of mats, boards or batts with an insulating R value ofat least 3.5/inch and preferably at least 4/inch. Specifically, theinvention relates to control of the drawn filament by a flow ofpressurized air flow parallel to the extruded filaments to provideattenuation of the filaments within an attenuation slot provided in alower portion of the apparatus.

BACKGROUND OF THE INVENTION

[0002] It has been proposed to produce polyester (e.g. PET) non-woveninsulating mats, constructed by melt-blowing techniques, having R valuesof 4.0 or more per inch with mats using substantially continuous fibersof 3-12 microns. However, mass production of high-loft batts suitablefor the insulation of building structures have not, in the past, provedeconomical in spite of extensive research efforts devoted to producingsuch environmentally friendly products.

[0003] PET non-woven fiber mats, specifically for insulating purposes,whether commercial or domestic, have been proposed using melt blowingdie equipment in which melted PET is extruded through a plurality ofnozzles to form substantially continuous fibers which are then carriedby a high velocity gas toward a fiber mat forming location, at which thefibers are laid down with self entanglement, resulting from the highlyturbulent accelerating gas flow, to produce the desired batt integrity.It is proposed in the art to produce such insulating batts (and boards)via one or more arrays of nozzles disposed in a straight line arrangedover the mat forming location to progressively produce the desired battconfiguration as it is conveyed under the array(s). As the fibers areextruded by the nozzles, they are collected on a collection device, in alayer of fibers to form the insulating mats, batts or boards.

[0004] U.S. Pat. No. 5,248,247 discloses an aligned nozzleconfiguration, two slot ducts producing air jets directed to intersect,at an acute angle, the spin line below the nozzle carrying die face (ornear it). The role of the air jets is to cause the extruded polymerfilaments to be stretched and expose the fibers to turbulent air flowand preferably broken up prior to deposition in a random mass on themoving belt below the die. The main thrust of the patent is directed atthe provision of a uniform driving pressure along the entire lateral dielength for the air supply system feeding the slot nozzles. It ispostulated, in this prior art, that even small variations, along the dielength, in this total driving pressure applied to the slot air flow willlead to an unacceptable non-woven product/mat.

[0005] Other components of the meltblowing die are elongate platesreferred to as air knives (nozzle bars), which form an acceleratingairflow channel to, in combination with the die tip nose piece, formconverging air flows to attenuate and draw down the extruded fibers tomicrosized diameters. The air knives are generally elongate plates whichhave a longitudinal edge tapered to form a knife edge. Two air knivesare typically used and are fastened to the face of the die body onopposite sides of the triangular die tip nose piece. The tapered edgesof the air knives are aligned with the confronting tapered surfaces ofthe nose piece and spaced slightly therefrom to form two flow channelswhich converge at the apex of the nose piece.

[0006] The spatial relationship between the air knives and the die tipis defined in the art by parameters known as air-gap and set-back. Theair-gap and set-back determine the geometry of the converging air flowpassages, and thereby control the airflow properties and the degree offiber-air interaction.

[0007] The prior art melt blowing apparatus as disclosed in U.S. Pat.No. 5,248,247 for production of melt blown filament line is showngenerally in FIG. 1 as comprising an extruder 1, melt blowing die 4 anda collector drum or conveyor belt 12. The extruder 1 delivers moltenresin through an aligned evenly spaced series of nozzles 6 in the die 4,where, upon exiting the nozzles 6, an aligned evenly spaced plurality offilaments (fibers) 2 are extruded to be attenuated and passed downthrough tapered slits, in a lower portion of the apparatus onto theconveyor beltl2, by pressurized, converging hot air streams. The taperedslit 11 is formed by adjacent parallel relatively thin nozzle bars 5through which the combined air/fiber stream passes. The filaments 2 arethen collected on the belt 12 to form a mat or fleece of insulation F.

[0008] The melt blowing apparatus also includes a source of pressurizedair 3 communicating with the die 4 through valved lines 8 and heatingelements 7 in order to produce the converging hot air streams 9.Additionally, baffles and air pressure regulating devices 10 areprovided together with the heating elements 7 and valved lines 8 tocontrol the conditioned hot air streams 9.

[0009] As is known to those in the art, the extruder 1 includes aninterior cavity where PET chips or similar polymer material arepressurized, heated and melted to produce the molten PET resin. Theextruder 1 is provided with the aligned evenly spaced plurality ofnozzles 6 communicating with the cavity. The nozzles 6 are supplied withmolten PET under pressure to form an aligned evenly spaced plurality offilaments 2 at a desired flow rate.

[0010] In conjunction with the molten resin flow, the hot air streams 9are provided from the pressurized air source 3 via the valved lines 8into confluence with the filament line 2 substantially adjacent thenozzle 6. The hot air streams 9 are directed by an outlet oriented so asto introduce each of the air streams into the slit 11 at an acute angleto the direction of the flow of the filaments 2, thereby attenuating anddrawing the filaments 2 downwards towards the conveyor belt 12 asillustrated in FIG. 1.

[0011] The slit 11 does not provide parallel flow controlling walls andis formed by the relative thin nozzle bars in which the slot formingwalls converge throughout the vertical height of the slot and therebyfail to provide a controlled flow of the air, passing therethrough,parallel to the filaments and thus do not provide adequate control offilament attenuation and temperature. Here no mention of controlling thetemperature of the slot walls is made.

OBJECTS OF THE INVENTION

[0012] It is an object of the present invention to provide an improvedmelt blowing apparatus and method to more effectively control theproperties of attenuated filaments formed thereby.

[0013] The main objective of the invention is to provide an attenuatingair flow in a vertical direction, parallel to the exiting filamentdirection so that appropriate shearing forces may be applied to theextruded filaments in the attenuating channel. This objective isachieved by means of a small radius Coanda bend or by a suitablydesigned channel flow, immediately upstream from the entrance to theattenuating channel.

[0014] Another object of the invention is to provide adequate fiberentanglement below the die face and the exit plane of the slotdischarge, by means of the highly turbulent flow field and the free airentrainment existing in this region.

SUMMARY OF THE INVENTION

[0015] Shearing forces applied to an extruding filament of moltensynthetic fiber material (polymer) by a suitably configured air/gas flowsystem provide an important means of influencing and controlling themolecular orientation, crystallinity and crystal orientation in certainhigh speed fiber spin line applications. The control of both themagnitude and location of the applied shearing forces, through thedesign characteristics of the air/gas system, is crucial to theachievement of improved and optimized mechanical properties of theresulting drawn fibers.

[0016] In contrast to the above described prior art, the presentinvention provides an accelerating high velocity fiber attenuating airflow in a vertical, attenuating slot extending along the fiber length asit is extruded. At the entrance section of this slot, along the fibercenter line, the extruded filaments move vertically downward with arelatively low velocity. In this slot surrounding these filaments isprovided an accelerating parallel high velocity air flow, with a maximumvelocity approximately two orders of magnitude greater than the emergingfilament velocity. The air flow is supplied by two identical, mirrorimage, ducting systems symmetrically disposed one on either side of thedie nozzle center line, with each incorporating a rapid turning sectionimmediately upstream of the slot entrance, so that the air flow entersthe attenuating channel flows in a substantially vertical (downward)direction. In the attenuating slot, shearing and attenuating forces andtemperature quenching are applied to the extruded molten filaments. Thefinal product's physical properties are critically dependent on themagnitude and time/space histories of the shearing and temperaturequenching applied.

[0017] At the exit of the attenuating slot, the air discharge from theattenuating slot emerges as a free turbulent jet quickly acquiring highturbulent energy levels. In particular, large lateral turbulent velocitycomponents are developed due to free air entrainment. The lattercontribute significantly to the entanglement of the now rapidlysolidifying or solidified filaments collected below the slot exit plane.

[0018] An important element of the present invention relates to thesupply of suitably conditioned attenuating air flow to the extruded(polymer) filaments. To develop the necessary shear forces at the airflow/polymer interface, the air flow must be delivered to the spunfilaments (fibers) in a substantially vertical direction (i.e. parallelto the length of the filaments), at a point as close to the exit of theextruded filament from the nozzles as possible. The air flow mustexecute a very tight turn, approaching 90°, to arrive at the verticaldirection at or near the top of the spin line, after traversing anapproximately horizontal path across the die extruding components (dienozzles) by which the filaments are extruded. A Coanda bend in the airsupply is a preferred means of achieving this separation free flowturning.

[0019] Two identical air flow channels symmetrically converge on the diecenter line at the top of the spin line, on either side of the extrudedfilaments. The converged air flows from the systems, together with theextruded filaments, enter an attenuating slot, where the main shearingforces and temperature quenching are applied to the molten filaments.The degree of temperature quenching is controlled by the temperaturedifference maintained between the extruded fluid filament, at the dieexit, and the conditioned attenuating air supply used.

[0020] Two alternative attenuating air supply systems are described tomeet the major design objectives/requirements of the present invention.These objectives are:

[0021] The mean air flow velocity must be increased significantly to ahigh subsonic value at the downstream delivery location at the top ofthe spin line. The outlet/inlet velocity ratio required in the airsystem is on the order of 10:1 to 20:1 with the exact value dependentupon the required filament shearing forces and the drawing/attenuationneeded in the final product.

[0022] The air flow must be turned to a substantially vertical dischargedirection, by means of a small radius of curvature turn, at orimmediately above the flow discharge into the attenuating slot.

[0023] The rapid turning and acceleration of the mean air flow in thesystem must be achieved without the introduction of any adverse flowpressure gradients on the walls defining the flow passages in the airsupply system.

[0024] The delivered high velocity air flow at the top of the spin linemust be uniform, along the length of the die, and uniformly across theinlet to the attenuating slot.

[0025] In the first of the general design approaches, FIG. 2 reveals asuitably configured and curved, fully attached internal flow channel todeliver the necessary air to the spin line. The air flow channel has ageneral “S” shaped center line contour, with the first, low speed turndirecting the air flow entering (approximately vertical) across thebottom of the high pressure polymer nozzle assembly towards the spinline. The second, high speed turning in the “S” channel orients thedischarge flow into the vertical spin line direction with a small radiusbend. The air flow acceleration in the channel is such that highaccelerations are applied in the low-velocity sections of the channel,including the first, low speed, turn, while small and vanishingaccelerations are applied in the high-velocity sections including thesecond, high speed, turn. The final high speed turn must be carried outusing a relatively small radius bend in order to permit the applicationof air shearing forces vertically at or near the top of the spin line.The entering air in the supply system is at a low velocity determined bythe supply ducting and the blower/fan/ compressor used to produce thenecessary supply of air pressure and volumetric capacity of the diesystem. The air supply system also includes a suitable air heating unitto provide appropriate control of the temperature in thedrawing/attenuating processes in and below the attenuating slot section.The final air discharge velocity from the supply system will typicallybe in the Mach No range between 0.50 and 0.75 (400-800 f.p.s.) althoughwider limits are not precluded.

[0026] In the second of the general design approaches, for the airsupply system (FIG. 4) the second turn described above which turns theair flow to the vertical spin line direction, is replaced by a short,approximately horizontal, wall jet section and a two-dimensional Coandabend of approximately 90°. The curved free surfaces of the wall jet andthe Coanda bend are vented to atmospheric pressure, as shown, through asuitable ducting arrangement. These free Coanda surfaces locatedsymmetrically on both sides of the spin line entrain a significantvolume of vented air prior to the convergence of the wall jets at thetop of the spin line, at the entrance to the final attenuating channelsection. On either side of the spin line trapped and standing vorticesmay be maintained above the curved free jet surfaces. Recirculation intothe flow volume containing the trapped vortices must be terminated by asuitable wall contour design, prior to the convergence of the two Coandawall jets at the entrance to the attenuating channel section. Coandawall turns provide excellent flow turning properties when properlydesigned and vented. With turning radius to jet thickness ratios in theregion 4-6, total turning angles of greater than 130° can be achievedwithout wall separation.

[0027] Acceleration rates of the air/gas flow in the discharge channelare set at levels appropriate to the desired axial strains to be appliedto the attenuating fiber filaments. The necessary flow accelerations areprovided through appropriate area and geometry variations incorporatedinto the discharge nozzle design.

[0028] Additional control of the drawn filament properties in thedrawing scheme described, can be obtained by adjusting and controllingthe temperature difference between the extruded polymer filament and thequenching air/gas flow utilized.

[0029] In certain applications, it may prove advantageous to provide thenecessary gas/air flow turning into the spin line direction, turningthis flow into the spin line direction, by combining a Coanda bendsection with a suitably curved fully attached channel flow section. Thusthe total required flow deflection would be achieved in separate, butconnected, channel sections.

[0030] A Coanda jet is a term applied to a class of jet flows having thefollowing features: i) a thin wall jet flow discharging over a straightor an arbitrarily curved wall surface, and in continuous contact withthis surface, at one edge (side), so that entrainment at this edge isentirely eliminated; ii) the remaining (outer) jet edge is exposed to aconstant pressure region when large free air entrainment occurs.

[0031] The feature of Coanda jet flows that is particularly attractivefor present design purposes is the relatively very tight wall curvaturesthat can be negotiated without the expected separation of the jet flowfrom the wall surface. The wall jet may be either laminar or turbulent,however, for present applications a laminar flow is preferred.

[0032] The most important inventive aspect of this submission wouldappear to be as follows: i) provision for an abrupt turn andacceleration of the attenuating air flow into the spin line direction,without wall separation to accomplish the required turning flow and ii)the application of the major attenuating forces to the filamentsinternally in an attenuating slot. The magnitude and axial variation ofthe magnitude of the applied shear forces are controlled by the designof the channel section and the temperature of the supplied attenuatingair flow. In particular the axial variation of the channel flow area isan important design consideration. For the formation of non-woven matsfrom PET the following parameters of the present invention are typical:

[0033] Extrusion die head temperature of 500/700° F.

[0034] Filament Velocity—exiting the polymer nozzle of about 0.1 toabout and exiting the die slot with a velocity in the range of about 20to about 200 feet per second. Large variations in both of these are tobe expected, with a factor of plus/minus, three/four quite probable(both depend on the die design objectives).

[0035] Air Flow Velocity—exiting the polymer nozzle ≈400/800 f.p.s.Again large variations can be expected (design dependent) with an upper(sonic) limit of approximately 1200/1400 f.p.s.

[0036] Filament Diameter Attenuation ratio 10:1 to 100:1.

[0037] Original Typical Filament Diameter of about 0.01-0.02 inch.

[0038] Attenuating Slot Width/Height

[0039] width—0.10-0.50 inch

[0040] height—0.25-2.50 inches

[0041] Die clearance above Table—2 to 20 ft. typical.

[0042] Temperature of Attenuating Air (Die Entrance) ˜500/700° F.,typical.

[0043] Temperature of Entrained Air from ambient ˜+50°.

[0044] Dies—heated ˜400/700° F., typical.

[0045] Two general design approaches for the air supply system requiredare sketched in FIGS. 2 and 4. FIG. 2 configuration does not incorporatethe Coanda effect of FIG. 4 to achieve the required flow turning. InFIG. 2, turning is accomplished via duct wall design, with the polymerexterior surface providing the inner duct wall profile. The air flow inthe case, is smoothly and rapidly accelerated, through a large areacontraction (10:1) by means of cubic wall profiles, and simultaneouslyturned into the spin line at the base of the polymer nozzle. A veryaccurately controlled wall profile is required throughout the length ofthe die, to avoid air flow separation in the resulting “S” shapednozzle.

[0046] According to the invention there is provided a melt blowing dieapparatus, for extruding a plurality of polymer filaments for themanufacture of non-woven thermally insulating polymer mats, comprising:a) a die having a downwardly facing die face, defining a plurality ofpolymer filament extruding nozzles having axes directed to extrude thefilaments vertically downwardly; b) a slot defined by vertical opposedparallel side walls evenly spaced on opposite sides of the axes, throughwhich the filaments, extruded by the die through the nozzles, pass; andc) a pair of air supply channels located adjacent the downwardly facingdie face, one on either side of the axes, each for the supply of a hotair stream vertically downwardly to and through the slot on oppositesides of said axes in contact with the filaments to attenuate thefilaments passing vertically downwardly through the slot thereby toproduce attenuated filaments to form the mats subsequent to downwardexit from the slot.

[0047] Also according to the invention there is provided a method ofmelt blowing polymer filaments, for the manufacture of non-woventhermally insulating polymer mats, comprising the steps of: a) extrudinga plurality of polymer filaments downwardly; b) passing the filamentscentrally through a slot, having vertical parallel slot defining sidewalls, common to all the filaments; c) providing heated air streams onopposite sides of the filaments, to flow vertically with the filamentsthrough the slot to attenuate the filaments while in the slot and belowto produce attenuated filaments for the formation of the mats.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The invention will now be described, by way of example, withreference to the accompanying drawings, in which:

[0049]FIG. 1 is a diagrammatic elevation of a melt blowing apparatusaccording to the prior art;

[0050]FIG. 2 is a partial cross-section of the nozzle and nozzle bardefining an attenuating slot according to the present invention;

[0051]FIG. 3 is a simplified diagrammatic underview of the apparatus ofFIG. 2; and

[0052]FIG. 4 is a partial cross-section of the nozzle and nozzle bardefining a Coanda bend and attenuating slot according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0053] Referring to FIGS. 2 and 3, a first embodiment of the presentinvention will be described. The melt blowing apparatus 20 includes anozzle bar 36 which in conjunction with the polymer die 22 defines asuitably configured and curved, fully attached air flow channel 24. Theflow channel 24 delivers a hot air stream 26 to an extruded spin line offilaments 28. The flow channel 24 has a generally “S” shaped contour,with the relatively large radius turn 30 directing the stream 26 acrossthe bottom of the die 22 towards the filaments 28. A relatively smallradius turn 32 in the air flow channel 24 orients the hot air stream 26to flow substantially parallel to the vertical direction of movement ofthe filaments 28 in a slot 34. The temperature of the attenuating airstream 12 as it reaches the filaments 28 is typically from about 500 toabout 700° F., and the walls of the slot are heated in a range of about400° F. to about 700° F.

[0054] The air stream in the channel 24 are such that high accelerationstake place in the large radius turn 30, while smaller accelerationsoccur in the high-velocity occurring in the small radius turn 32. Thefinal turning of the hot air stream 26 is carried out by the relativelysmall radius bend 32 at the entry of the slot 34 in the nozzle bar 36 inorder to obtain the desired application of air shearing forces at ornear the top of the filaments 28 adjacent to the uniformly spaced arrayof filament extruding nozzles 38 of the die 22.

[0055] The air entering the flow channel 24 from a pressurized airsupply source is at a low velocity determined by the supply ducting andthe blower/fan/compressor used to produce the necessary supply of airand volumetric capacity of the air supply system. The air supply systemincludes an air heating unit to provide appropriate control of thetemperature in the drawing/attenuating processes in and below theattenuating slot 34. The air discharge to the slot 34 from the air flowchannel 24 is typically in the Mach number range between 0.50 and 0.75although wider limits are not precluded.

[0056] The nozzle bar 36 also defines the attenuating slot 34 throughwhich the filaments 28, downwardly extruded by nozzles 38, are drawndownwardly by the attenuating air flow. The attenuating slot 34 has alength (FIG. 3) extending laterally of the filaments symmetrically alongeither side of the serial plurality of nozzles 38 to provide asymmetrical consistent attenuating air flow to each of the plurality offilaments.

[0057] The major attenuating forces comprise both axial and shear forcesapplied to the filaments by the air flow. These forces are generated andcontrolled internally in the attenuating slot 34. The axial attenuationof the filaments 28 is dependent upon the magnitude of the forcesapplied to the filaments 28 in the slot 34 and these forces arecontrolled by the configuration of the flow channel 24 and theattenuating slot 34, in particular the axial velocity distribution andthe axial temperature distribution in the parallel air therethrough.

[0058] The nozzle bar 36 and attenuating slot 34 are formed from a firstand second lower nozzle plates 42, 44 having a parallel first and seconddie faces 46, 48 defining the slot 34 and symmetrically disposed onopposite sides of the filaments 28. The nozzle plates 42, 44 areprovided with heater coils 45 to provide desired temperature control inthe slot 34. The first and second die faces 46, 48 are spaced from oneanother to define the attenuating slot width W in the range of about0.100 to about 0.50 inch. The height H of the attenuating slot, which isgenerally in the range of about 0.250 to about 2.50 inches, includes theheight h of the parallel first and second die faces 46, 48 of theattenuating slot 34 which are generally in the range of about 0.18inches to about 2.0 inches.

[0059] From the extruder apparatus 2, molten polymer 40 is forceddownwardly through the nozzles 38 to form the filaments 28, having adiameter, as they leave the nozzle, in the range of about 0.01 to 0.02inch. The attenuating forces, both axial and shear, generated by theattenuating air flow 26 in the attenuating slot 34 to attenuate thediameter of the filaments 28 in a range of at least approximately 50:1before the filament exits the attenuating slot 34 to be gathered on acollection belt.

[0060] As will be seen in FIG. 2, the lower face of the die 22 closelyadjacent opposite sides of the row of nozzles 38 has a concave formwhich, together with the corresponding curves in nozzle plates 42, 44form the small radius turns 32.

[0061] Turning now to FIG. 4, a second embodiment of the presentinvention is described. Here, each second turn 32 described above, forturning of the air stream 26 to flow in the vertical filament linedirection, is replaced by a short, approximately horizontal, wall jetsection and duct 50 and a Coanda bend 52 of approximately 90° having anassociated duct 54 open to atmospheric pressure of the environment. Theduct 54 provides a supply of air 56 to become entrained in a hot airstream supplied by duct 50 to produce the desired Coanda effect of airflow around the curved free surfaces of the Coanda bend 52.

[0062] The duct 56 is separated from the wall jet section and duct 50 byan intermediate bar 58 which provides for the separate introduction ofthe air 56 from the duct 54 and the pressurized air stream 26 from duct50, to the entrance to the Coanda bend 52, in the form of a thin walledjet flow emanating from the duct 50. The thin walled jet flow exitingfrom the horizontal section wall jet section and duct 50 has an upperboundary which is exposed to the constant pressure via the ductingarrangement 54 from which free jet entrainment occurs. A lower boundaryof the thin walled jet flow discharging from the duct over the curvedfree surfaces 60 of the Coanda bend 52 by the Coanda effect is caused toremain in continuous contact with the lower curved free surface 60 inorder to obtain the desired turning of the pressurized hot air stream 26around the small radius turn of the Coanda bend 52 into alignment withthe filament line direction of movement. The temperature of theentrained air is generally ambient air at a temperature of about 50° F.or more.

[0063] The free Coanda surfaces 60 are located symmetrically on oppositesides of the extruded filaments 28. On either side of the nozzles,trapped and standing vortices may be maintained above the curved freejet surfaces. Recirculation into the flow volume containing the trappedvortices must be terminated by a suitable wall contour design, prior tothe convergence of the two Coanda wall jets at the entrance to theattenuating slot 64.

[0064] Coanda bends provide excellent flow turning properties whenproperly designed and vented. With turning radius to jet thicknessratios in the region 4˜6:1 total turning angles of greater than 130° canbe achieved without wall separation.

[0065] Nozzle bars 62 which define the free surfaces 60 together formattenuating slot 64 through which the filaments 28 are drawn down by theattenuating air flow. The attenuating slot 64 has a length extendingsymmetrically along either side of the plurality of nozzles 38 toprovide a symmetrical consistent attenuating air flow to each of theplurality of filaments.

[0066] The major attenuating forces comprise both axial and shear forcesapplied to the filaments by the air flow. These forces are generated andcontrolled internally in the attenuating slot 64. The axial variation ofthe filaments 28 is dependent upon the magnitude of the shear forcesapplied to the filaments and these forces are readily controlled by theconfiguration of the duct 50 and the attenuating slot 64, in particularthe width W and height H of the attenuating slot 64, together with theform of the Coanda bend 52.

[0067] The attenuating slot 64 is formed by parallel faces 65 of nozzlebars 62 which faces 65 smoothly transitioning from the outlet ends ofthe Coanda bends 52. The faces define the slot width W in the range ofabout 0.10 inch to about 0.50 inch. The height H of the faces 65 definethe height H of the attenuating slot 64, which is in the range of about0.25 inch to about 2.5 inches.

[0068] From the extruder 2, the molten polymer 40 is extruded throughthe nozzles 38 forming the filaments 28, having a diameter, as theyleave the nozzle, in the range of about 0.01 to 0.02 inch. Theattenuating forces, both axial and shear, generated by the attenuatingair flow applied to the filaments 28 within the attenuating slot 64attenuate the diameter of the filaments 28 at least in a range ofapproximately 50:1 before the filament exits the attenuating slot 64 andis gathered on a collection belt 10.

[0069] Molten polymer is supplied at a suitably elevated temperature, tothe nozzles 38, and filaments 28 are discharged uniformly, verticallydownward by a suitable pressurized supply system. Air/gas streams areintroduced laterally from both sides. These gas streams are deflectedinto the spin line direction by means of two-dimensional Coanda bends(90°, as shown). The curved free jet surface, at the outer edge of theCoanda bend, entrains and accelerates the individual cylindricalfilaments 28 discharged vertically above it. Once the air/gas streamsare deflected into a direction parallel to the filaments' downwardmovement, the flow provides further important axial acceleration to thefluid filaments as the streams merge to form a single vertical dischargeto atmosphere at the lower die face. This latter acceleration isattributable to the large axial shear forces applied to the attenuatingfluid elements in the discharge slot. The applied shearing forces are aresult of the large axial velocity difference maintained between thefilaments and the air/gas stream. (The mean axial air/gas velocity inthe discharge channel is approximately two orders of magnitude largerthan the initial discharge velocity of the fluid filaments.)

[0070] Acceleration rates of the air/gas flow in the discharge channelare set at levels appropriate to the desired axial strains to be appliedto the attenuating fiber filaments. The necessary flow accelerations arereadily provided through appropriate area and geometry variationsincorporated into the discharge nozzle design.

[0071] Additional control of the drawn filament properties in thedrawing scheme described, can be obtained by adjusting and controllingthe temperature difference between the extruded polymer filament and thequenching air/gas flow utilized.

[0072] In certain applications, it may prove advantageous to provide thenecessary gas/air flow direction, turning this flow into the spin linedirection, by combining a Coanda bend section with a suitably curvedfully attached channel flow section. Thus the total required flowdeflection would be achieved in separate, but connected, channelsections.

[0073] The air flow through the slot is preferably lamina, however, thepossible use of turbulent flow in the slot is not excluded from theconcept of the present invention.

[0074] The air leaving the slot is or becomes rapidly turbulent withlarge turbulent energy levels which applies important lateral forces tothe emerging attenuated filaments to facilitate the desired entwinementof the fibers to produce the non-woven mats, batts or boards constructedupon collection of the filaments on the belt 10.

REFERENCE NUMERALS

[0075]1 extruder

[0076]2 filaments

[0077]3 source of air

[0078]4 die

[0079]5 nozzle bars

[0080]6 nozzles

[0081]7 heating elements

[0082]8 valved lines

[0083]9 hot air streams

[0084]10 baffles

[0085]11 slit

[0086]12 belt

[0087]20 melt flowing apparatus

[0088]22 die

[0089]24 air flow channel

[0090]26 hot air stream

[0091]28 filaments

[0092]30 large radius turn

[0093]32 small radius turn

[0094]34 slot

[0095]36 nozzle bar

[0096]38 extruding nozzles

[0097]40 molten polymer

[0098]42 nozzle plate

[0099]44 nozzle plate

[0100]45 heating coils

[0101]46 die face

[0102]48 die face

[0103]50 duct

[0104]52 Coanda bend

[0105]54 associated duct

[0106]56 air

[0107]58 intermediate bar

[0108]60 free surface

[0109]62 nozzle bars

[0110]64 slot

[0111]65 parallel faces

1. A melt blowing die apparatus, for extruding a plurality of polymerfilaments for the manufacture of non-woven thermally insulating polymermats, comprising: a) a die having a downwardly facing die face, defininga plurality of polymer filament extruding nozzles having axes directedto extrude the filaments vertically downwardly; b) a slot defined byvertical opposed parallel side walls evenly spaced on opposite sides ofthe axes, through which the filaments, extruded by the die through thenozzles, pass; and c) a pair of air supply channels located adjacent thedownwardly facing die face, one on either side of the axes, each for thesupply of a hot air stream vertically downwardly to and through the sloton opposite sides of said axes in contact with the filaments toattenuate the filaments passing vertically downwardly through the slotthereby to produce attenuated filaments to form the mats subsequent todownward exit from the slot.
 2. The apparatus of claim 1, wherein thenozzles are disposed in an evenly spaced straight line array and theslot is common to all nozzles in the array.
 3. The apparatus of claim 1,wherein the air supply channels are identical in mirror image and eachbounded, at least adjacent the nozzles, by the die face and a nozzle bardefining the slot.
 4. The apparatus of claim 3, wherein the air supplychannels each decrease in cross-section to accelerate the air streamflowing therethrough to a desired velocity for supply to and throughsaid slot.
 5. The apparatus of claim 4, wherein the channels each have arelatively large radius bend remote from said slot providing arelatively high acceleration of the air stream and a relatively smallradius bend, adjacent said slot to orient the air stream to flowsubstantially vertically into and through the slot, providing relativelysmall acceleration of the air stream.
 6. The apparatus of claim 5,wherein the relatively small radius bend is a Coanda bend.
 7. Theapparatus of claim 3, wherein each channel has a Coanda bend therein todirect the hot air stream passing therethrough vertically into the slot.8. The apparatus of claim 7, wherein each channel has a duct for theintroduction of ambient air, to the hot air stream adjacent thedownwardly facing die face at the entrance to the Coanda bend forentrainment with the upper boundary of the hot air stream to ensurecontinuous contact of the hot air stream with a curved surface definedby the nozzle bar and smoothly joining said side walls to provide saidhot air stream in said slot.
 9. The apparatus of claim 1, wherein theslot has a transverse width of about 0.15 inch to about 0.30 inch and aheight of about 1.0 inch to about 2.5 inches; the air streams in theslot have a velocity of about Mach no. 0.5 to about 0.75.
 10. Theapparatus of claim 1, wherein the channels and slot are configured toprovide laminar flow of the air streams in the slot at the desired thevelocity.
 11. A method of melt blowing polymer filaments, for themanufacture of non-woven thermally insulating polymer mats, comprisingthe steps of: a) extruding a plurality of polymer filaments downwardly;b) passing the filaments centrally through a slot, having verticalparallel slot defining side walls, common to all the filaments; c)providing heated air streams on opposite sides of the filaments, to flowvertically with the filaments through the slot to attenuate thefilaments while in the slot to produce attenuated filaments for theformation of the mats subsequent to exit from the slot.
 12. The methodof claim 11 comprising directing the heated air streams to flowvertically through the slot by the use of Coanda bend.
 13. The method ofclaim 11 comprising providing a laminar flow of the air streams throughthe slot.
 14. The method of claim 11 comprising providing an air flowthrough the slot which becomes turbulent upon exit from said slot toimpart large lateral accelerations to the filaments subsequent to theexit from the slot to facilitate the required fiber entanglement. 15.The method of claim 11, wherein the height of the slot as defined bysaid side walls provides for the attenuation of the filaments by the airstreams in the slot by at least 50:1.
 16. The method of claim 11comprising providing the air streams as they contact the filaments atabout 500° F. to about 700° F. and heating the slot side walls to atemperature of about 400° F. to about 700° F.